The present invention generally relates to an optical intensity modulator, and more particularly to an improvement in an optical intensity modulator which modulates an incident laser beam on the basis of an electric field applied thereto.
Referring to FIG. 1, there is illustrated a conventional optical intensity modulator. The illustrated optical intensity modulator is made up of an n.sup.+ -type compound semiconductor substrate 1, an n.sup.- -type active layer 2, a p.sup.+ -type compound semiconductor layer 3, an electrode 4 formed on a bottom surface of the n.sup.+ -type semiconductor substrate 1, and electrode 5 formed on a surface of the p.sup.+ -type semiconductor layer 3. An incident laser beam L emitted from a laser diode (not shown) is projected onto a first end surface of the optical intensity modulator. The incident laser beam L penetrates the active layer 2, which serves as a waveguide path. Then the penetrated laser beam is emitted from a second end surface of the optical intensity modulator which is opposite to the first end surface.
Generally, the width of the forbidden band in the active layer 2 is set 20-30 [meV] larger than an amount of energy of the incident laser beam L. When a reverse bias voltage is applied between the electrodes 4 and 5, the absorption edge in the active layer 2 expands toward the low energy side due to the Franz-keldish effect. Thus, the incident laser beam L is absorbed within the active layer 2, and no laser beam is emitted from the second end surface of the modulator. In this manner, the incident laser beam L is intensity-modulated by turning ON/OFF the reverse bias voltage to be applied to the pn junction between the n.sup.- -type active layer 2 and the p.sup.+ -type semiconductor layer 3.
However, the conventional optical intensity modulator presents a disadvantage in that the incident laser beam L is somewhat absorbed in the active layer 2 even when the reverse bias voltage is turned OFF. This disadvantage will be understood with ease from a distribution of an electric field in the pn junction.
FIG. 2 is a graph illustrating a distribution of an electric field in the pn junction. The horizontal axis of the graph represents distance measured in the direction of thickness of each layer, and the vertical axis thereof represents intensity of the electric field [V/cm]. In FIG.2, the same reference numerals as those shown in FIG. 1 indicate the same elements shown in FIG.1. A solid line indicated b V.sub.0 is a characteristic line which shows a distribution of the electric field. The characteristic line relates to a case where the active layer 2 is 2000 [.ANG.] thick. It can be seen from the graph that an electric field is being applied to the active layer 2 due to the presence of a diffusion voltage even when the reverse bias voltage is set equal to zero. This means that the absorption end in the active layer 2 expands toward the low energy side and thus the incident laser beam L is absorbed in the active layer 2. As a result, it is impossible to draw a laser beam having a sufficient power from the second end surface.
FIG.3 is a graph illustrating expansion of the absorption edge. The horizontal axis of the graph represents energy, and the vertical axis thereof represents an absorption coefficient. A solid line is a characteristic line obtained in the absence of electric field, and a broken line is a characteristic line obtained when the reverse bias voltage is applied between the electrodes 4 and 5. A reference letter P.sub.AB indicates the absorption edge in the active layer 2, and .DELTA.E is the difference in energy between the absorption edge P.sub.AB and the incident laser beam L. It can be seen from the graph of FIG. 3 that intrinsically there is the definite energy difference .DELTA.E between the absorption ed P.sub.AB and the incident laser beam L in the absence of electric field. Thus, it is expected that the incident laser beam L emitted from the modulator has an intensity identical to a design value. However, as described previously, the absorption edge P.sub.AB expands when a diffusion voltage exists in the active layer 2, and the incident laser beam L is absorbed and attenuated in the active layer 2.
In order to overcome the aforementioned shortcoming, it is conceivable to set the difference .DELTA.E in energy between the absorption ed P.sub.AB and the incident laser beam L equal to or greater than about 50 [meV]. However, the above setting presents another disadvantage in that a change in the absorption coefficient obtained when the reverse bias voltage is applied is small.